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Hybrid 0D-2D nanoheterostructures: in-situ growth of amorphous silver silicates dots on g-C3N4 nanosheets for full spectrum photocatalysis Shouwei Zhang, Huihui Gao, Xia Liu, Yong-Shun Huang, Xijin Xu, Njud S. Alharbi, Tasawar Hayat, and Jia-Xing Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09260 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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ACS Applied Materials & Interfaces

Hybrid 0D-2D nanoheterostructures: in-situ growth of amorphous silver silicates dots on g-C3N4 nanosheets for full spectrum photocatalysis Shouwei Zhang†a, Huihui Gao†a, Xia Liub, Yongshun Huangb, Xijin Xu*a, Njud S. Alharbid, Tasawar Hayat d, Jiaxing Li*b,c,d a

School of Physics and Technology, University of Jinan, Shandong, 250022, P. R.

China. E-mail: [email protected] b

Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics,

Chinese Academy of Sciences P.O. Box 1126, 230031 Hefei, P. R. China Tel: +86-551-65596617, Fax: +86-551-65591310, E-mail: [email protected] c

Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education

Institutions, P.R. China d

NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia. †

These authors contributed equally to this work.

Key words: Photocatalysis; 0D-2D; g-C3N4; Ultradispersed amorphous silver silicates; Interfacial engineering

ACS Paragon Plus Environment


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Abstract: The smaller particle sizes, better dispersion and more heterojunction interfaces can enhance the photocatalytic performance of photocatalysts. Herein, ultradispersed amorphous silver silicates/ultrathin g-C3N4 nanosheets heterojunction composites (a-AgSiO/CNNS) with intimate interfacial coupling effect were synthesized through the facile in situ precipitation of ultrafine a-AgSiO (~5.2 nm) uniformly dispersed on the entire surface of hierarchical ultrathin CNNS. In this process, the ultrathin CNNS not only perform as the support to form heterostructures, but are employed as dispersant to confine the aggregation of a-AgSiO nanoparticles. Notably, the optimum photocatalytic activity of a-AgSiO/CNNS-500 composite is ~36 and 13 times higher than that of CNNS towards the degradation of rhodamine B and tetracycline, respectively. The excellent photocatalytic activity can be attributed to the synergistic interactions of heterojunction with strong interfacial coupling effect, improved visible light absorbance, abundant heterojunction interfaces and fully exposed reactive sites, which originate from the well-defined nanostructures such as uniform packing of the ultrasmall a-AgSiO, the intimate and maximum coupling interfaces between a-AgSiO and CNNS. We believe that such an easy and scalable synthetic strategy can be further extended to the fabrication of other ultrafine semiconductor coupled with g-C3N4 for increasing its photocatalytic performance.


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1 Introduction The application of semiconductor-based photocatalysts for the decomposition of toxic and hazardous organic pollutants is of great importance to solve the environmental pollution.1-3 However, the photocatalytic activities are still far from satisfactory due to the traditional UV-responsive photocatalysts and/or low photocatalytic efficiencies.4 Therefore, the development of novel visible-light responsive photocatalysts with high photocatalytic efficiencies have become an urgent work.5 Since the photocatalytic water splitting over g-C3N4 was reported by Wang et al. in 2009, it has attracted extensive attentions in photocatalytic degradation of organic pollutants because of its relatively narrow bandgap (~2.7 eV), reliable stability and low cost.6-8 However, g-C3N4 suffers from relative high photogenerated electron-hole recombination and weak visible light absorption, which led to the relative low photocatalytic efficiency.9-12 To overcome these disadvantages, numerous strategies have been developed.11, 13-16 For example, a series of g-C3N4 based photocatalysts were constructed by Dong group, which possessed highly enhanced visible-light photocatalytic performance for NO removal.17-19 Among these, coupling with other semiconductors with suitable bandgap to form heterojunctions is an effective way to extend light absorption ranges and to promote photogenerated charge separation and transfer.20-23 Various

silver-based materials/g-C3N4

heterostructures, such as Ag/g-C3N4,

g-C3N4/Ag2O, Ag/AgBr/g-C3N4, g-C3N4/Ag3PO4 and g-C3N4/AgIO3, have been developed to improve photogenerated charge separation rate and to enhance



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photocatalytic efficiencies.24-29 Our group has also developed silver-based-g-C3N4 heterojunction photocatalysts, which exhibited enhanced visible-light photocatalytic activities due to the extended light absorption range and excellent separation efficiency of photogenerated charge.30,

31

Recently, a series of amorphous silver

silicate photocatalysts were constructed by Huang group via a simple one-step co-precipitation method, which exhibited high light response and narrow bandgap and was considered as promising visible light photocatalysts.32-35 Among these advantages, the narrow bandgap is of particularly importance because it endows wide light response ranges to ensure harvest visible light, which is a prerequisite for achieving high photocatalytic efficiency.36 Therefore, it is expected that the amorphous silver silicate coupled g-C3N4 would exhibit high photocatalytic activity due to the reasons: (i) it has wide visible light absorption by broadening the light absorption spectrum towards the near infrared (NIR) light; (ii) it has suitable band-edge positions, which could form heterojunction, reduced photogenerated charge recombination and improved photocatalytic performance.37 As we known, the separation of photogenerated charge is an interface-based process.38 Abundant heterojunction interface will provide more separation channel for photogenerated electron-hole pairs to ensure more effective separation.4, 39 Thus, it is reasonable to expect that the photocatalytic activities of g-C3N4-based photocatalysts may be further enhanced by adjusting the nanoparticles (NPs) size and distribution of semiconductor coupling with g-C3N4.40-42 Adjusting the NPs sizes and distribution could endow semiconductors two advantages: (i) the smaller NPs significantly


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decrease the diffusion length of photogenerated charges, thus leading to increased photocatalytic performance; (ii) ultradispersed NPs over large exposed g-C3N4 surface can form more effective heterojunction, and the synergistic effect between semiconductors and g-C3N4 can be further improved. That is, the smaller particle sizes, better dispersion and more heterojunction interfaces can enhance the photocatalytic performance.22, 43-45 Herein, we demonstrated the construction of novel ultradispersed amorphous silver silicates/g-C3N4 nanosheets heterojunction nanocomposites (a-AgSiO/CNNS) with intimate interfacial contact via a facile in situ precipitation by uniformly dispersing ultrafine a-AgSiO on the surface of CNNS. In this process, the ultrathin g-C3N4 nanosheets will not only perform as the support to form heterostructures, but are also employed as dispersants to confine the aggregation of a-AgSiO NPs, leading to ultrafine a-AgSiO NPs uniformly packed throughout the surface of CNNS. The photocatalytic activities of a-AgSiO/CNNS heterojunctions were evaluated by degrading rhodamine B (RhB) and tetracycline (TC) under UV and visible light (VL) irradiation. Furthermore, the structure properties were discussed and the degradation mechanism was also proposed. 2 Experimental sections 2.1 Preparation of the amorphous silver silicates (a-AgSiO) As illustrated in Figure 1A, the a-AgSiO was synthesized via a facile precipitation method.32, 33 For a typical synthesis, AgNO3 (0.51 g) was dissolved in deionized water (100 mL) under stirring, followed by adding Na2SiO3·9H2O (0.29 g) within 1.0 h. The



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precipitates were collected and washed with distilled water and ethanol, and dried at room temperature. 2.2 Synthesis of g-C3N4 nanosheets (CNNS) Urea (30 g) was put in covered crucible and heated under static air at 550 oC for 4 h with a ramping rate of 2.5 oC/min. The powder was collected, placed in an open crucible and further heated at 500 oC for 2 h with a ramp rate of 5 oC/min. After cooling to room temperature, the desired ultrathin CNNS was obtained by washing with deionized water and further dried in a vacuum oven.46 2.3 Synthesis of amorphous silver silicates/g-C3N4 nanosheets composites (a-AgSiO/CNNS) As illustrated in Figure 1B, the a-AgSiO/CNNS composites were synthesized via an in situ precipitation method, similar as the synthesis of a-AgSiO. Specifically, AgNO3 (0.51 g) was dissolved in 100 mL of deionized water. Then, a certain amount of CNNS were added into the solution and sonicated for 2 h to disperse CNNS. After that, Na2SiO3·9H2O (0.29 g) was added into the suspension and the reaction was continued another 1.0 h. The precipitates were collected and washed with distilled water, and dried at room temperature. The a-AgSiO/CNNS composites prepared with different weights of CNNS in 50, 100, 200, 300, 400, 500 and 600 mg are denominated as a-AgSiO/CNNS-50, a-AgSiO/CNNS-100, a-AgSiO/CNNS-200, a-AgSiO/CNNS-300,

a-AgSiO/CNNS-400,

a-AgSiO/CNNS-500

a-AgSiO/CNNS-600, respectively. 2.4 Characterization


and

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Powder X-ray diffraction (XRD) data were collected using a D/MAX2500V diffractometer using Cu Kα radiation (λ = 1.5418 Å). The structural information of the samples were measured by a Fourier transform spectrophotometer (FT-IR, Avatar 370, Thermo Nicolet) using the standard KBr disk method. X-ray photoelectron spectroscopy (XPS) was performed using ESCALAB250 with Mg Kα as the source and the C 1s peak at 284.6 eV as an internal standard. The morphologies and compositions were characterized using a JEOL-2100 field emission transmission electron microscope (FETEM) at an accelerating voltage of 200 kV. UV-vis diffuse reflection

spectroscopy

(DRS)

was

performed

on

a

Shimadzu

UV-2500

spectrophotometer using BaSO4 as the reference. 2.5 Evaluation of photocatalytic activity Photocatalytic degradation experiments were performed in a Pyrex photocatalytic reactor equipped with a 500 W Xe lamp and a 400 nm cutoff filter as the light source. For a typical photocatalytic reaction, the photocatalysts (50 mg) were added into a RhB or TC solution (50 mL, 10 mg/L). The mixed solution was stirred for 60 min in the darkness to ensure the adsorption-desorption equilibrium. During the photocatalytic test, a small portion of the suspension (~3.0 mL) was obtained, and the catalytic efficiency was estimated on a UV-vis spectro-photometer (UV-2550, Shimadzu) at certain time intervals. 3 Results and discussion A facile in situ precipitation synthesis method was designed to the preparation of a-AgSiO and a-AgSiO/CNNS hybrid hierarchical three-dimensional architectures, and



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the preparation process is illustrated in Figure 1. The reaction process may be described by the Equation as follows: Ag+ + SiO32 − + OH− → AgXSiOY + H2O

For the preparation of a-AgSiO, Na2SiO3 was added to the AgNO3 solution with the formation of a-AgSiO. CNNS can be easily obtained by thermal oxidation etching of bulk g-C3N4 in air. The dispersed CNNS were negatively charged, with a zeta potential of about -27.6 mV (Figure S1).47 By adding CNNS into the AgNO3 solution, cationic Ag+ will bound tightly to the surface of CNNS via electrostatic interactions. Then, with the addition of Na2SiO3 solution, the fixed Ag+ cation will further react with SiO32- to generate a-AgSiO, resulting in a-AgSiO/CNNS.9, 28 In this case, CNNS may not only act as the support to form heterostructures, but are also employed as dispersants to confine the aggregation of a-AgSiO NPs, thus leading to ultrafine a-AgSiO NPs uniformly distributed on the surface of CNNS. This synthetic route to a-AgSiO/CNNS photocatalysts is economic, facile and can be produced in a large scale. 3.1 Material characterization The XRD patterns of pure CNNS, a-AgSiO and a-AgSiO/CNNS composites are shown in Figure 2. For the XRD pattern of pure CNNS, the diffraction peaks at 13.2o and 27.5o corresponded to (002) and (100) diffraction planes of graphitic carbon nitride, respectively.46 For a-AgSiO, a wide peak at ~33.7

o

was detected, no other

diffraction peaks can be observed, suggesting its amorphous structure.32 This peak was also detected in the a-AgSiO/CNNS composites, but decreased with increasing


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CNNS contents. On the contrary, the characteristic peaks intensity of CNNS increased as the CNNS contents increased. Both characteristic peaks from a-AgSiO and CNNS were detected in a-AgSiO/CNNS composites without any other peaks, indicating the successful synthesis of a-AgSiO/CNNS composites with high purity through in situ precipitation method. FTIR spectra of a-AgSiO, CNNS and the a-AgSiO/CNNS composites were collected in Figure 3. Compared with a-AgSiO, strong absorption bands within 1000-1800 cm-1 were found for a-AgSiO/CNNS composites, which belonged to the C-N stretching vibrations, while the broad band at 3000-3700 cm-1 was indicative NH stretching vibration modes.37 Moreover, the characteristic peak at ~814 cm-1 was assigned to the stretching vibration modes from the s-triazine ring unit, which confirmed the successful introduction of CNNS in the composites.48 Note that, after introducing the a-AgSiO, red shift was observed for the adsorption bands in the composites (Figure 3B).5, 49 Take the a-AgSiO/CNNS-500 composite as an example, the characteristic peaks located at ~1640, 1470, 1410, 1323 and 1240 cm-1 were shifted to lower wavenumbers (Figure 3B). The red shift of these bands suggested the weaken bond strengths of C=N and C-N due to the conjugation between CNNS and a-AgSiO. This result indicated strong interfacial coupling effect in the a-AgSiO/CNNS composites. Compared with the aggregated or large size NPs, the ultradispersed nature of a-AgSiO NPs on CNNS acted as nano-islands could facilitate the formation of the heterojunction interfaces and guaranteed higher contact areas.41 Both characters are essentials to promote the separation efficiency of photogenerated charge and to



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enhance photocatalytic activity. The surface chemical compositions of CNNS, a-AgSiO and a-AgSiO/CNNS-500 composite were investigated by XPS (Figure 4). As shown in Figure 4A, the XPS survey spectra indicated the presence of C, N in CNNS, Ag, Si, O, C, and N in a-AgSiO/CNNS-500 composite, and Ag, Si, O in a-AgSiO, respectively. The C 1s spectra of a-AgSiO/CNNS-500 and CNNS were shown in Figure 4B. The C 1s characteristic peaks originated from sp2-bonds of graphitic carbon (C-C, C=C) (~284.4 eV) and defect-containing sp2-bonded carbon (N-C=N) (~288.2 eV) in the composite shifted to higher binding energies as compared to CNNS.47 High resolution N 1s peaks were also presented in Figure 4C. Three obvious peaks at ~398.0, ~399.1 and ~400.7 eV were observed, corresponding to sp2-hybridized N (C-N=C), tertiary N (N-(C)3) groups and amino functional groups (C-N-H). These peaks also shifted to higher binding energies in the composite. High resolution Ag 3d spectrum of a-AgSiO showed two individual peaks at ~374.3 and 368.3 eV, corresponding to the Ag 3d3/2 and Ag 3d5/2, respectively (Figure 4D). After combined with CNNS, the Ag 3d peaks of a-AgSiO/CNNS-500 displayed lower binding energies as compared to a-AgSiO (Figure 4D). The Si 2p spectrum indicated the binding energy of Si 2p centered at ∼103.2 eV, which was a typical value for metal silicate hydroxides (Figure 4E). The O 1s XPS spectra of a-AgSiO/CNNS-500 had two peaks located at ~530.2 and 531.7 eV, respectively, while only one peak was observed for a-AgSiO (Figure 4F). Correspondingly, the Si 2p and O 1s peaks in the a-AgSiO/CNNS-500 composite also shifted towards the lower binding energies as compared with pure a-AgSiO. Based on


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the above analysis, the peaks of Ag 3d and Si 2p from a-AgSiO had lower binding energies in the composite, while higher binding energies of C 1s and N 1s from CNNS were observed in the composite. These phenomena can be explained by partial electron transfers from CNNS to a-AgSiO, i.e., increase (decrease) the electron density of AgSiO (CNNS) leads to the reduction (enhancement) of the binding energies of Ag 3d and Si 2p (C 1s and N 1s), which further indicated the presence of strong electronic interactions between CNNS and a-AgSiO, promoting the photogenerated charge separation and transfer. From the XPS and FTIR results, we can expect the strong interfacial coupling effect between CNNS and a-AgSiO, which could promote photogenerated electron-hole pairs separation and transfer, and further enhance the photocatalytic performance of a-AgSiO/CNNS composites. The light absorption properties of the CNNS, a-AgSiO and a-AgSiO/CNNS were investigated by UV-vis DRS (Figure 5A). The light absorption edge of CNNS was measured to be ~452 nm, corresponding to a band gap (Eg) of ~2.74 eV, which is slight blue shifted with respect to that of bulk g-C3N4. Nearly all visible lights can be absorbed by a-AgSiO with an absorption edge of ~810 nm, corresponding to a band gap of ~1.53 eV. The a-AgSiO/CNNS composites presented the hybrid absorption features of both a-AgSiO and CNNS. Compared with CNNS, all a-AgSiO/CNNS composites exhibited broader visible light absorptions due to the heterojunctions formed between a-AgSiO and CNNS, which changed the optical properties of the composites. The intensities of the absorption bands increased with increasing amount of a-AgSiO, which were consistent with the color change of the samples (Figure 5B),



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which turned from light yellow to red brown with the increase of a-AgSiO. It is inferred that the light absorption capabilities of CNNS could be significantly improved by heterojunction via loading a-AgSiO as “nano-island” on the surface of CNNS, resulting in significantly decreased interfacial contact barrier and strengthened electronic coupling of the semiconductors to generate more photogenerated electrons/holes with improved photocatalytic performance. The morphologies and microstructures of CNNS, a-AgSiO and a-AgSiO/CNNS-500 were investigated by FETEM (Figure 6). CNNS were observed as thin nanosheets with wrinkles, in which overlapped layers were depicted as black stripes (Figure 6A). Similar morphologies can be seen for a-AgSiO/CNNS-500 with crumpled nanosheets, numerous wrinkles and folds (Figure 6B). High resolved TEM images (Figure 6C and D) illustrated the uniform deposition of a-AgSiO NPs on the surface of CNNS with relative narrow size distributions of ~5.2 nm in diameter. No free a-AgSiO NPs or naked CNNS were observed. These ultradispersed a-AgSiO NPs could provide maximum interfacial contact with the CNNS surface, which could further strengthen synergistic coupling effect between a-AgSiO NPs and CNNS. Moreover, the intimate contact between a-AgSiO and CNNS would further strengthen the photogenerated charge separation and transfer. Compared with traditional aggregated or large contact structures, this ultrasmall a-AgSiO NPs on hierarchical CNNS three-dimensional structures could not only provide more surface active sites for sequential photocatalytic reactions, but also shorten the migration distance of photogenerated charges.


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The morphologies of composites with different CNNS contents are shown in Figure 7. It should be mentioned that partial aggregations were observed for composites with lower CNNS contents (50, 100, 200, 300 and 400 mg, Figure 7B-F), while uniformly distributed a-AgSiO NPs were found for higher CNNS contents (500 and 600 mg, Figure 7G-H). To reveal the roles of CNNS in confining a-AgSiO NPs growth, the same preparation procedure was applied to fabricate a-AgSiO NPs without CNNS, resulting in large diameters of a-AgSiO NPs (30-50 nm), indicating the key roles of CNNS in the fabrication of ultrasmall a-AgSiO NPs (Figure S2). The decreased particle sizes with increased CNNS content clearly indicated that CNNS could not only perform as the support to form heterostructures, but are also employed as dispersants to confine the aggregation of primary a-AgSiO NPs. The uniformly dispersed a-AgSiO NPs on the entire surface of CNNS could be further verified by the elemental mapping and EDS images (Figure 8), where the Ag, Si, O, C and N elements were homogeneously distributed over the whole profile, demonstrating the successful coupling of a-AgSiO with CNNS to form heterostructures. The atomic ratio of the Ag/Si/O in AgSiO calculated from the EDS is approximately 3:1:6 (Table S1) 3.2 Photocatalytic tests To evaluate the photocatalytic activities of a-AgSiO/CNNS composites under UV and VL irradiation, RhB and TC were chosen as model pollutants for photocatalytic degradation (Figure 9). Negligible self-degradation was observed for RhB and TC under VL irradiation (Figure 9B and D), while the degradation reached ~7.6 % and



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~4.7 % under UV irradiation for ~40 min, respectively. As expected, all a-AgSiO/CNNS composites exhibited higher photocatalytic activities than either a-AgSiO or CNNS by both UV and VL irradiations with a hierarchical order of a-AgSiO/CNNS-500

>

a-AgSiO/CNNS-600

>

a-AgSiO/CNNS-400

>

a-AgSiO/CNNS-300

>

a-AgSiO/CNNS-200

>

a-AgSiO/CNNS-100

>

a-AgSiO/CNNS-50, indicating the positive effect of CNNS contents to enhance the photocatalytic activities of the composites. Full degradation of RhB were observed within ~15 and 140 min by UV and VL irradiation in the presence of a-AgSiO/CNNS-500 composite, respectively (Figure 9A and B), illustrating the significantly improved photocatalytic activity of the ultradispersed a-AgSiO NPs/CNNS composites. However, further increment of CNNS (a-AgSiO/CNNS-600) resulted in decreased photocatalytic activity, which may be attributed to the recombination of photogenerated electrons and holes to restrain the photocatalytic efficiency. To further exclude the dye self-sensitized process during the photocatalytic reaction, colorless and poor self-sensitized TC was selected for degradation by a-AgSiO/CNNS composites (Figure 9C and D). The degradation efficiencies of TC were ∼97.5% and ∼90.5% for the a-AgSiO/CNNS-500 under UV and VL irradiation within ~40 and ~140 min, respectively. These degradation efficiencies were much higher than that of pure CNNS (∼56.2% for UV and ∼23.8% for VL) and a-AgSiO (∼26.6% for UV and ∼14.6% for VL). The results clearly indicated the excellent photocatalytic degradation efficiencies of a-AgSiO/CNNS composites for organic pollutants.


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The excellent photocatalytic performance a-AgSiO/CNNS composites could be attributed to the synergistic interactions from the following aspects: (i) the ultrafine a-AgSiO NPs can shorten diffusion distance of photogenerated charge to the surface; (ii) the ultradispersed nature of a-AgSiO NPs on CNNS will form strong interfacial coupling effects, which can not only promote the photogenerated charge migrate, but also improve the stability of the nanocomposites; (iii) the hierarchical nanosheet structures can provide increased reactive sites for organic pollutants adsorption and degradation.42 To investigate the reaction kinetics of RhB and TC degradation, the experimental data were analyzed by the pseudo-first-order kinetic model,50  C - ln   C0

  = Kt 

where K (min-1) is the rate constant, C0 (mg/L) is the initial concentration of RhB or TC, and C (mg/L) is the concentration of RhB or TC at time t (min). Figure S3 and S4 depicted the linear relationships of all these photocatalysts. Under UV and VL irradiations, the highest photodegradation rates for RhB (TC) were calculated to be ~0.3314 min-1/0.0284 min-1 (0.0798 min-1/0.0116 min-1) for a-AgSiO/CNNS-500, respectively,

which

~4.5/12.3(3.4/5.5),

were

~12.1/35.5

~3.5/9.2(2.9/4.1),

(4.36/12.8),

~39.9/40.6

~2.7/6.6(2.2/2.9),

(11.5/8.92),

~2.9/6.2(1.9/2.5),

~2.8/4.2(1.7/2.1) and ~2.2/2.1(1.4/1.4) times higher than those of CNNS, a-AgSiO, a-AgSiO/CNNS-50, a-AgSiO/CNNS-100, a-AgSiO/CNNS-200, a-AgSiO/CNNS-300, a-AgSiO/CNNS-400 and a-AgSiO/CNNS-600. To further illustrate the advantages of the as-prepared photocatalysts, the rate constants, K values, were compared before



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and after being normalized with the surface area (Table S2). The activity enhancement decreased from ~15.9 to ~9.7 times after being normalized with the surface area, further suggesting the synergistic effects of both increased photogenerated charges and high surface area on the degradation enhancement. Figure 10A and B showed the variations of the absorption spectra of RhB and TC under VL irradiation by using a-AgSiO/CNNS-500, respectively. The characteristic peaks intensities of RhB and TC gradually decreased by prolonging the irradiation time, and the adsorption peaks disappeared within ~140 min irradiation. The corresponding digital photographs of RhB degradation by using different photocatalysts under different irradiation times were collected and displayed in Figure 10C. Recycling experiments were also performed on a-AgSiO/CNNS-500 to evaluate the stability of this photocatalyst (Figure 11A). Within the same irradiation time (~140 min) more than ~95 % RhB were degraded even after seven successive cycles, illustrating its high stability and great promise in practical applications.51 To evaluate the structural stability, the surface components and composition of a-AgSiO/CNNS nanocomposite before and after photocatalytic reaction were investigated by XPS analysis. As shown in Figure S5A and S5B, peaks of Ag0 3d are present in the XPS spectra of the samples, and the content of Ag0 is about 3%, of the total Ag after six cycles, respectively. The repeated photocatalytic reactions were also carried, the activities of the photocatalyst in the recycle time six decrease somewhat, which can be attributed to the formation of Ag0 on surface and the loss of samples in the recycle


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experiment. The formed Ag0 reduces the specific surface area and light absorption of samples, resulting in the decrease in the activity of a photocatalytic reaction. In the Figure S5C, a lower Ag0 peak was observed in its XRD spectra pattern. That is, the stability experiment results confirmed that the a-AgSiO/CNNS photocatalyst exhibited broad spectrum photocatalytic degradation ability and stability, but the inevitable presence of photocorrosion in the case of long working hours. Although the photocatalysts have some photocorrosion after six cycles, the incorporation of CNNS with AgSiO photocatalyst can not only enhance the visible light photocatalytic performance, but also inhibit the photo-corrosion in a large extent. Therefore, a-AgSiO/CNNS composite is a promising photocatalyst for the removal of organic pollutants in the environmental protection. 3.3 Photocatalytic mechanism The organic pollutants can be effectively degraded by reactive species, including h+, ·OH and ·O2-, which may vary for different photocatalysts due to their different band structures and phase compositions.50 Therefore, to explore the mechanism of the high photocatalytic activity and to assess the contribution of the reactive species, trapping experiments of reactive species were conducted using ethylenediaminetetraacetate (EDTA-2Na), iso-propyle alcohol (IPA) and benzoquinone (BQ) as h+, ·OH and ·O2scavenges, respectively.27, 49 By adding these scavenges into the degradation solutions, the reactive species in the degradation process can be revealed, as illustrated in Figure 11B for the degradation of RhB by a-AgSiO/CNNS-500. While no photocatalytic efficiency change was observed by adding BQ, slightly decreased photocatalytic



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efficiency was obtained with the addition of IPA, indicating no effect of ·O2- species and slight effect ·OH species for RhB degradation. Degradation of RhB was dramatically suppressed by introducing EDTA-2Na, indicating the major active species of h+ in the degradation process. The electron spin resonance (ESR) spin trap technique, using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin-trapping reagent, is well-known to be an efficient measurement method to determine ·OH. Under visible light irradiation, it clearly appeared a fourline spectrum with the relative intensities of 1:2:2:1 in Figure S6, which was considered to be the characteristic spectrum for the DMPO-·OH adduct. While, no obvious ESR signals were observed in the dark time. This was consistent with the results that ·OH was major oxidation active species. PL spectra are usually used to explore the efficiency of the photogenerated charge trapping, migration, and transfer.52 A low PL intensity is generally indicative of a high separation efficiency of photogenerated electron-hole pairs. Figure 11C showed PL spectra of CNNS, a-AgSiO and a-AgSiO/CNNS composites under excitation wavelength of 360 nm at room temperature. A strong emission peak of CNNS was observed at ~460 nm. With increasing CNNS contents, the PL intensities of the a-AgSiO/CNNS composites decreased significantly as compared to CNNS. Generally speaking, the weakened PL intensity suggested the decreased luminous recombination probability of its photogenerated charge and thus was favorable for promoting the photogenerated charge separation and transfer via intimate interfacial contact charge transfer to improve its photocatalytic activity. Figure 11D displayed a comparison of


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the photocurrent-time (I-t) curves for CNNS, a-AgSiO and a-AgSiO/CNNS-500 that were recorded over four on-off cycles of intermittent VL irradiation.53 The fast photocurrent transient responses for on-off cycles could be observed, and it can be clearly seen that the a-AgSiO/CNNS-500 composite displayed the largest photocurrent as compared to that of CNNS and a-AgSiO under VL illumination, which could be ascribed to the existence of the maximum interfacial contact between a-AgSiO and CNNS, where photogenerated electrons and holes could be efficiently separated, thus leading to the enhanced photocatalytic activity of a-AgSiO/CNNS-500 composite. The valence bands (VB) of both CNNS and a-AgSiO were measured by XPS valence spectra (Figure 12A). The position of the VB edge of CNNS and a-AgSiO were located at +1.36 and +2.01 eV, respectively. Combined with the calculated bandgap (Figure 5A), the conduction band (CB) edges of CNNS and a-AgSiO were calculated to be at -1.38 eV and +0.48 eV, respectively. The extraordinarily high activities of the heterostructured photocatalysts should possess a good separation of photogenerated electron-hole pairs at the heterojunction interfaces. Based on the CB/VB edge potentials, the schematic diagrams of band structure of different composites are depicted in Figure 12B. Under VL irradiation, both CNNS and a-AgSiO could absorb visible light, resulting to the excitation of e- to the CB while h+ remains in the VB. For the heterojunction, the photogenerated e- in CB of CNNS could easily migrate to the CB of a-AgSiO, and at the same time the h+ generated in the VB of a-AgSiO could migrate to CNNS. Namely, the suitably aligned band edges of CNNS and a-AgSiO



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suggested that effective photogenerated charge migrate may occur through the heterojunction with strong interfacial coupling effect in the composite. The conclusion can be further confirmed by the XPS and FTIR results. Therefore, the restriction of photogenerated e- and h+ on the different sides via photogenerated charge migration reduces the recombination rate of photogenerated electron-hole pairs and increases the abundance and stability of the photogenerated charge in the composite, resulting in improved photocatalytic efficiencies. The potential of the O2/·O2- couple is less negative than the CB potential of CNNS, but more negative than the CB potential of a-AgSiO. Therefore, the e- generated in the CB of CNNS can react with O2 to form active ·O2- species in the photogenerated electron migration process from the CB of CNNS to the CB of a-AgSiO in composites, while the e- generated in the CB of a-AgSiO cannot activate O2 to generate ·O2-. It is reasonable that minor effect of active ·O2- species was observed in photocatalytic process since most of the electrons will effectively transfer from the CB of CNNS to the CB of a-AgSiO in composites, which is confirmed by the radical trapping experiments (Figure 11B). The photogenerated holes are collected in the VB of CNNS, which cannot directly oxidize OH- or adsorb H2O to form ·OH (E0 (OH-/·OH) = +1.99 eV, E0 (H2O/·OH) = +2.7 eV).49 However, ·OH can be generated via O2 multi-electron transfer routes, in accordance with the more negative potential of the CB on a-AgSiO, which further confirmed the photocatalytic results obtained by using IPA as a scavenger (Figure 11B). Finally, the photogenerated holes collected in CNNS can diffuse to the composite surfaces by directly oxidizing organic pollutants due to their


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high oxidation potentials, which were confirmed by the photocatalytic results from EDTA-2Na as a scavenger (Figure 11B). Based on the above analysis, photogenerated h+ and ·OH are the primary active species to determine photocatalytic performance, while the enhanced photocatalytic activity is attributed to the efficient photocatalytic charge separation driven by the well-matched band-structures of a-AgSiO and CNNS. 4 Conclusions In summary, we have successfully developed a series of a-AgSiO/CNNS composites via a facile in situ precipitation method. The ultrathin g-C3N4 nanosheets showed hierarchical morphology and the diameters of a-AgSiO nanoparticles were within the range of 3-8 nm. CNNS not only perform as the support to form heterostructures, but are also employed as dispersants to confine the aggregation of a-AgSiO NPs. The as-synthesized a-AgSiO/CNNS-500 photocatalyst showed superior visible light photocatalytic activities than others, which can be ascribed to the synergetic effect between a-AgSiO and CNNS, including the maximum heterojunction interface with intimate contact, enhanced photogenerated charge separation efficiency, fully exposed reactive sites as well as excellent visible light response in the composite. This work could give insights into the importance of rational design of heterojunction systems, and provide a potential method for the construction of efficient heterojunction photocatalysts with controllable sizes and space distributions. ASSOCIATED CONTENT Supporting Information: Zeta potential of ultrathin CNNS dispersed in water, SEM image of a-AgSiO and the pseudo first-order rate constant of the degradation of RhB



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and TC under different conditions, The XPS spectra and the XRD pattern of a-AgSiO/CNNS-500 before and after photocatalytic reaction, etc. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected] Author Contributions †

These authors contributed equally to this work.

Acknowledgements Financial supports from NSFC (Grant No. 51672109, 21272236, 11304120, 21225730, 2167716), and the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, the Priority Academic Program Development of Jiangsu Higher Education Institutions and Deanship of Scientific Research (DSR) References 1.

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Figure 1. Schematic illustration of the synthesis of (A) a-AgSiO and (B) a-AgSiO/CNNS composites.



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Figure 2. XRD patterns of the CNNS, a-AgSiO and a-AgSiO/CNNS composites.


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Figure 3. (A) FT-IR spectra of CNNS, a-AgSiO and a-AgSiO/CNNS composites and (B) the magnified curves in the range of 2000 cm-1 to 600 cm-1.



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Figure 4. (A) The survey XPS spectra of CNNS, a-AgSiO and a-AgSiO/CNNS-500 composite; (B) and (C) high resolution spectra of C 1s and N 1s for CNNS and a-AgSiO/CNNS-500 composite; (D), (E) and (F) high resolution spectra of Ag 3d, Si 2p and O 2p for a-AgSiO and a-AgSiO/CNNS-500 composite.


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Figure 5. (A) UV-vis diffuse reflectance spectra and (B) digtal photographs of CNNS, a-AgSiO and a-AgSiO/CNNS composites.



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Figure 6. The FETEM images of the CNNS (A) and a-AgSiO/CNNS-500 (B-D). The inset of (D) depicts the histogram showing the a-AgSiO size.


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Figure 7. FETEM images of (A) pure CNNS and the a-AgSiO/CNNS composites synthesized with different contents of CNNS: (B) 50 mg; (C) 100 mg; (D) 200 mg; (E) 300 mg; (F) 400 mg; (G) 500 mg and (H) 600 mg.



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Figure

8.

The

elemental

mapping

images

and

EDS

a-AgSiO/CNNS-500 composite.


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spectrum

of

the

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Figure 9. The photocatalytic activities of the as-prepared photocatalysts for the degradation of RhB and TC under UV (A and C) and VL (B and D) irradiation.



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Figure 10. The absorption spectra of RhB (A) and TC (B) degraded by a-AgSiO/CNNS-500 composite under VL irradiation; (C) the corresponding digital photograph of RhB degraded by a-AgSiO/CNNS composites under VL irradiation.


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Figure 11. (A) Stability of a-AgSiO/CNNS-500 composite for RhB degradation under VL irradiation; (B) Radical trapping experiments during the photocatalytic degradation of RhB over a-AgSiO/CNNS-500 composite under VL irradiation; (C) PL spectra of CNNS and a-AgSiO/CNNS composites; (D) Transient photocurrent response for pristine CNNS, a-AgSiO and a-AgSiO/CNNS-500 composite.



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Figure 12. (A) Valence band spectra of CNNS and a-AgSiO; (B) Proposed mechanisms of photogenerated charge transfer and pollutants degradation in the a-AgSiO/CNNS composites under VL irradiation.


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Graphical Abstract


Hybrid 0Dâ2D Nanoheterostructures: In Situ ... - ACS Publications

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